In conventional manner, the core of such a reactor is loaded with more than a hundred juxtaposed nuclear fuel assemblies.
Each assembly comprises a bundle of nuclear fuel rods, the rods each having a cladding which surrounds nuclear fuel pellets.
The operation of a nuclear reactor is carried out in the form of cycles, at the end of which the nuclear fuel assemblies of the core are replaced, for example, in thirds.
In this manner, at the end of an operating cycle, for example, one third of the nuclear fuel assemblies present in the core are removed, two thirds of the nuclear fuel assemblies in the core are moved and a new third of nuclear fuel assemblies are introduced so that they are subjected to the following operating cycle.
Conventionally, the loading map, that is to say, the arrangement of the nuclear fuel assemblies within the core, is selected from a plurality of loading maps envisaged in order to produce the maximum amount of energy during the future operating cycle. This selection is brought about, for example, by the operator of the nuclear reactor, using computer programs which involve neutron calculation software.
By way of example, documents EP-786 782 and U.S. Pat. No. 7,224,761 describe methods for neutron optimization of loading maps.
After determining a loading map which is acceptable on the basis of neutron criteria, it must be verified that, the loading map envisaged does not involve any risk of exceeding the long-term thermomechanical limits within the nuclear fuel assemblies. Exceeding such limits in this manner could lead in particular to occurrences of thermal instability within the core.
This verification is generally provided by the designer of the fuel assembly taking into account a notional rod which would be subjected to a local power level greater than all the local power levels of the rods, regardless of the time of the cycle in question; this is referred to as an envelop power history. The duration of such a verification is in the order of from two to three days. After the loading map selected on the basis of neutron criteria is confirmed with respect to the thermomechanical criteria, the operator of the nuclear power station produces a report showing compliance with the safety parameters of the reactor so that the safety authorities definitively validate the loading map selected.